Urchin turning white

I got a long spine urchin about 6 months ago and it was jet black in the LFS. The next day I noticed some white banding on a few of the spines and then over the next few weeks it got much more pronounced.

My first thought was water too. The water parameters are good right now but they haven’t always been that way (high po4) so it’s possible. I guess I was hoping that if it was the water that it would get better over time with better water but I haven’t noticed any change (better or worse).